Bluetooth: Protect request_firmware with a semaphore for btusb

If btusb_probe gets called during system suspend/resume,
request_firmware call will fail sometimes as userspace processes
that help in request_firmware are frozen as a part of suspend.

Change-Id: I51c6fc0221e56e9dfa507aee0c26005e805a9e78
Signed-off-by: Rupesh Tatiya <rtatiya@codeaurora.org>